Cladistic Tests of Hypotheses Concerning Evolution of Xerophytes and Mesophytes Within Tillandsia Subg Phytarrhiza (Bromeliaceae)

Tillandsia L. Subg. Phytarrhiza (Visiani) Baker (Bromeliaceae) is a distinctive group of about 35 epiphytic species. These exhibit a range of habits from xeric to mesic. The evolutionary relationships of the contrasting adaptations need to be established here as well as in the subfamily as a whole. Relations between the subgenus and other tillandsioids are problematical and phylogenetic reconstruction of its member-species would be facilitated by identification of Phytarrhiza's relative (sister taxon) sharing the same most recent common ancestor with Phytarrhiza. This paper examines the two most likely sister taxa, Subg. Pseudo-Catopsis Baker and Subg. Diaphoranthema (Beer) Baker. Diaphoranthema is rejected as sister taxon. The accepted evolutionary tree, rooted by Pseudo-Catopsis, indicates that most habital evolutionary changes in Phytarrhiza have been between mesic and semi-mesic forms and from mesic to xeric forms. Methods developed for testing specific evolutionary hypotheses are broadly applicable. THERE IS CONSIDERABLE interest in the direction of evolution of species within Phytarrhiza as well as for the entire genus and subfamily because of questions raised by Pittendrigh (1948), Medina (1974), and Benzing and Renfrow (1971 a, b), and discussed by Benzing, Givnish and Bermudes (1985), and Gilmartin (1983), regarding relationships between mesic, tank forms and more strees-adapted "atmospheric bromeliads." Benzing et al. (1985) examined the question: are extreme xeric tillandsioids derived from mesic formsSchimper's (1888) interpretation?; or are mesic, epiphytic tillandsioids derived from xeric precursors-Pittendrigh's (1948) interpretation? Benzing also considered a third interpretation, proposed by Medina (1974) who examined carbon pathways. Medina proposed that both xeric and mesic tillandsioids arose from precursors adapted to conditions of high light intensity and humidity. There is little support for the often held notion that xeric adaptations in general, are more frequently acquired from mesic progenitors than the reverse. Some reasons for the notion are the many examples in the world's deserts ' Received for publication 6 May 1985; revision accepted 4 October 1985. Research supported by NSF Grant BSR 84407573 to the authors. The following individuals provided extremely helpful critical reviews: David Benzing, Phil Cantino, Vicki Funk, Loren Rieseberg, Karen Simmons, Douglas Soltis, and John Utley. Robin Lesher provided technical assistance and plant illustrations were rendered by Sheila Gilmartin. ofxeric specializations of otherwise mesic taxa; for example, coreaceous, linear leafed species of Lycium (Solanaceae). Such xeric adaptations might seem to be difficult to reverse, i.e., unlikely to evolve to a more mesic state. Stebbins (1974) pointed out, however, that any belief that xerophytes, in general, are irreversibly specialized has no foundation in fact. We are left with both courses being essentially equally